DESCRIPTION: Second and third generation ultrasonic contrast agents have created an exciting new opportunity to image local microvascular perfusion. Physicians and physiologists have long recognized the significance of tissue perfusion, however, important questions remain unanswered due to the lack of assessment tools. Our initial observations show promise for a new family of diagnostic tools that could be applied to assess cancer, cardiovascular disease, glaucoma and wound healing. The techniques described in this proposal could provide the opportunity to map microvascular density and heterogeneity at the correct scale for the detection of tumors and to assess the maximum local microvascular transit time. Using a unique new experimental system, we have established that a set of physical phenomena affect the scattered signal from ultrasound contrast agents, and provide significant opportunities for new tools for the assessment of vascular function as well as structure. In this application, we propose to extend these studies to evaluate these effects through intravital microscopy, and to take advantage of these phenomena to tailor a new imaging mode to assess vascular density and microvascular transit time. Due to our expertise in tumor vascularity, we propose to focus our investigations on applications in this area. However, the proposed techniques could have far wider clinical application. We propose to map vascular density and transit time. A new mode called recovery imaging will be employed in which microbubbles are destroyed in a local region and the timing for echo restoration determined. These new methods will be evaluated in phantoms, through intravital microscopy in an ischemic rat model, and in a final comparative study of four mouse tumor cell lines and a benign fibroadenoma model.